PNX1502E Datasheet, PDF(313/819 Page) NXP Semiconductors – Connected Media Processor

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PNX1502E Datasheet, PDF (313/819 Pages) NXP Semiconductors – Connected Media Processor

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Philips Semiconductors

Volume 1 of 1

PNX15xx Series

Chapter 9: DDR Controller

Whenever a CPU DDR burst is started with dynamic ratios, the internal account is

incremented by an amount equal to âthe number of clock cycles spent on the previous

CPU DDR burstâ times the CPU_RATIO. This way the real overhead is measured and

accounted for and therefore the accounting mechanism is much more accurate.

CPU account

CPU_CLIP

CPU_LIMIT

#cycles_in_burst

#cycles_in_previous_burst

x CPU_RATIO

slope = CPU_DECR/cycle

transfers

constant average account below clip,

see text

time

Figure 5: CPU account using dynamic ratios

In the time zone marked âconstant average account below clipâ in Figure 5, the

transfer rate is such that the average value of the CPU account is constant. In this

zone, we have the following equilibrium:

CPU_RATIO Ã #cycles_in_previous_burst = CPU_DECR Ã (#cycles_between_arbitration â #cycles_in_burst)

Where #cycles_in_burst is the actual number of cycles it takes to complete this DDR

burst, and #cycles_in_previous_burst is the total number of clock cycles spent on

executing CPU bursts since the previous CPU command won the arbitration.

From this the CPU bandwidth with constant average account is derived:

CPU_BW = #----c--y---c---l--e-#-s--c-_-y--b-c--e-l-t-e-w-s---_e---ie-n--n-_--_-b--a-u-r--br--s-i--tt--r--a--t--i--o---n- = C-----P---U----_---R----A-----T---I--O------Ã-----#-------c-----y------c-----l-#---e---c--sC---y--_---P-c--i---n-Ul---e--_---_s---p--_-D---r--i--e-n-E---v--_--C--i--b--o--R--u--u----r--s--s--_---t---b------u------r----s-----t---+-----C----P---U-----_---D----E----C----R--

Considering that on average, the number of cycles in a burst will be equal to the

number of cycles in the previous burst, the average CPU bandwidth is:

average(CPU_BW) = --------------------C----P----U----_---D----E----C----R----------------------

CPU_RATIO + CPU_DECR

When the CPU account exceeds the CPU_CLIP value, the account can only stay

constant when the CPUs take 100% of the available bandwidth. This is unlikely to

happen because the CPU account exceeds the CPU_LIMIT value, giving DMA a

higher priority than the CPUs. Therefore, the CPU account will not stay above

CPU_CLIP for long.

With dynamic ratios enabled, the free bandwidth is also handled differently. When the

bandwidth is for free i.e., the account is above the CPU_CLIP value, the internal

account is not incremented at all. To ensure the internal account has the same value

before and after the free bandwidth DDR burst, the account never decrements

12NC 9397 750 14321

Product data sheet

Rev. 2 â 1 December 2004

Â© Koninklijke Philips Electronics N.V. 2002-2003-2004. All rights reserved.

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